Once you've compiled it for one platform, you've re-bootstrapped it, at which point you can use the real compiler to cross-compile for another platform.
Ideally, you want more than one bootstrapped platform. Platforms eventually die, and you don't want to rely on an emulator for bootstrapping.
Some time from now x86_64 will fade away, and there's a large chance rust will still be around. I know that this will probably take a long time, but it's better and easier to do it now than later.
